Mechanical filters composed of cascaded flexure-mode bars and coupling wires are known: Y. Tomikawa, D. P. Havens, R. A. Johnson and S. Sugawara, "Resonances in Flexure-Mode Mechanical Filters", Proceedings of 1976 Ultrasonics Symposium, Sept., 1976, pp. 597-601; M. Konno, S. Sugawara, Y. Tomikawa and R. A. Johnson, "Mounted Free-Free Flexural Bar Resonator and Mechanical Filter", Proceedings of International Symposium on Circuits and Systems, June, 1979, pp 1068-1071; and R. A. Johnson "Mechanical Filters Take On Selective Jobs", Electronics, Oct. 13, 1977, pp. 81-85. The coupling wires are located on a single plane, such as the top or the bottom surfaces of the resonator bars. These filters suffer from problems of inadequate strength, in the case of narrow filter bandwidths, and from spurious response problems, in the case of wide filter bandwidths.
The narrow bandwidth filter requires small diameter coupling wires to achieve the requisite small acoustic coupling between the resonators. The small diameter wires lack the physical strength to support the filter under high shock and vibration conditions.
Wide bandwidth filters require larger diameter coupling wires than narrow bandwidth filters. The increased coupling wire diameter raises the frequencies of the filter spurious responses, which are due to rigid-body modes. In some filter designs, these higher spurious frequencies fall close to the filter passband, degrading filter performance. In addition, the effect of these rigid-body modes is more pronounced in the wideband case because the stopbands and transition bands of the wide passband filters are wider and are more likely to be in the region of the spurious responses. The effect of these rigid-body modes is to often cause unwanted amplitude variations in the stopband, transition band, and passband of the filter.
Previous attempts to reduce spurious responses have involved the size and position of coupling wire welds, the addition of a third wire in the plane of the two torsional mode coupling wires, and variations of coupling wire and support wire lengths, as shown in the above noted references. Attempts to improve strength characteristics have involved increasing the bar resonator size, changing resonator spacing, and the use of higher order modes of vibration. These attempts to reduce spurious response effects and to increase strength have resulted in marginal improvements. Unfortunately, these small improvements are commonly negated by variations in manufacturing processes.
There is a need for a substantial, quantum level improvement in increased stength and in reduced spurious response. There is a further need for a simple and cost effective solution, particularly with regard to manufacturability.